Bacterial cytokinesis is mediated by the septal ring (SR), a cytoskeletal-like organelle that forms at the future site of cell fission and that remains associated with the leading edge of the invaginating envelope during the process. The long-term goals of this project are to understand at a molecular level: i) The composition and architecture of the SR, ii) How the SR assembles from its different components, iii) How the proper site for its assembly is determined, iv) How the SR drives cell fission, and v) How SR function is coordinated with other cell cycle events. In E.coli, the SR consists of at least ten essential division proteins as well as multiple proteins with redundant and/or non-essential activities. SR development involves assembly of an early Z-ring (ZR) intermediate, recruitment of additional proteins to form a fission-competent SR, and further modifications after the onset of the fission process. Proper positioning of the SR at midcell is crucial to ensure equipartition of mothercell components. This is controlled by the Min system, by SlmA-dependent nucleoid occlusion (NO), and by an elusive third system that collectively dictate proper placement of the ZR. The ZR consists of polymers of the tubulin-like GTPase FtsZ that are decorated by at least four other division proteins. To understand how FtsZ polymers form a membrane-associated ring, we will determine the requirements for ZR assembly in vivo, and develop in vitro systems to monitor co-assembly of ZR proteins in loaded phospholipid vesicles. SlmA is a DNA-binding protein that prevents aberrant ZR assembly over nucleoids. How it does this is not clear, and we will search for DNA and/or protein partners of SlmA that help it to inhibit assembly of ZR proteins in vivo and in vitro. ZR positioning in cells that lack both Min and SlmA is still biased to membrane sites in between nucleoids. To obtain a molecular handle on this phenomenon we will implement genetic screens for mutants that have lost this bias. We have and will identify new proteins that join the SR at/after the onset of fission, and that play redundant roles in separating the in-growing murein layer and/or in invaginating the outer membrane. We will use genetic approaches and microscopy to elucidate how these proteins join the SR, and what their specific roles in the fission process are. A number of SR proteins are actively pursued as potential drug targets. This project may lead to the identification of additional promising targets for, and rational designs of, new anti-bacterial drugs.